1. Field of the Invention
The present invention relates to a furnace for producing a high purity quartz glass preform for an optical fiber. More particularly, it relates to a heating furnace for thermal treatment such as dehydration, doping or sintering of a porous glass preform consisting of fine particles of quartz glass in order to obtain a high purity quartz glass preform for fabricating the optical fiber.
2. Description of the Related Art
In a heating furnace for producing a glass preform for use in the fabrication of an optical fiber, use of a quartz glass muffle tube is described in, for example, Japanese Patent Kokoku Publication Nos. 42136/1983 and 58299/1983 and Japanese Patent Kokai Publication No. 86049/1985. However, the quartz glass tube has a serious disadvantage in that it tends to deform at a high temperature. In practice, when the heating furnace is operated at a temperature greater than 1500.degree. C., the quartz glass tube deforms so that the furnace cannot be used again, unless a pressure difference between the outside and the inside of the muffle tube and a means for supporting the muffle tube are both accurately controlled. Further, when the furnace is used at a temperature greater than 1150.degree. C. over an extended period of time, the quartz glass tube is devitrified or crystallized. Since the thermal coefficient of expansion of the glass layer is different from that of the devitrification layer, the muffle tube is destroyed by the strain resulting from such a difference.
The inventors have found that a carbon tube is suitable for a muffle tube and overcomes the above problems (see, for example, PCT International Publication No. WO 88/06145). The carbon tube not only has an excellent heat resistance since it is stable at a temperature greater than 2000.degree. C., but also is easily purified to a high purity level with an ash content less than 20 ppm. In addition, it advantageously does not react with a reactive gas (for example, Cl.sub.2, CCl.sub.4, SiF.sub.4, SF.sub.6 and CCl.sub.2 F.sub.2) which is useful for the thermal treatment of the glass preform for the optical fiber. The carbon tube can be fabricated precisely so that it may be made in an assembly type to reduce the production cost thereof. Further, in order to improve gas tightness of the carbon tube, it is possible to coat the outer surface thereof with a SiC layer or a further carbon layer whereby the glass preform for the optical fiber having an excellent quality can be produced.
FIG. 1 shows one example of the conventional heating furnace wherein the thermal treatment of a glass soot preform 1 is carried out with a cylindrical zone heater. A carbon heater 4 and a muffle tube 3 are provided in a furnace body 5. This heating furnace comprises an inlet 6 for introducing nitrogen gas for purging the furnace body interior, an inlet 7 for introducing an atmosphere gas to the muffle tube and a supporting rod 2 for the preform 1 which is placed inside the heating furnace. The muffle tube 3 consists of an upper member 34, a middle member 35 and a lower member 36 and at least the middle member 35 is made of carbon, on the surface of which a SiC or carbon coating may be provided.
Since the conventional heating furnace is constituted as shown in FIG. 1, an amount of air around the furnace (an atmosphere in the operation room) flows in the muffle tube when the glass preform enters or leaves the tube. FIG. 2 schematically shows an equipment which is used in measurement of an amount of the air inflow into the muffle tube. This equipment comprises a muffle tube 101, an inlet 102 for purging gas, a gas sampling tube 103, a device 104 for measuring an oxygen concentration and a pump 105, and additionally a zone heater (not shown) around the muffle tube 101. An inner diameter of the muffle tube 101 is 150 mm, and the front end of the gas sampling tube 103 is fixed at a point 1 m below from the upper edge of the muffle tube. The results are shown in FIG. 3. These results indicate that the air flows into the muffle tube, and such air inflow cannot be prevented by increase of the amount of purging nitrogen gas.
Inflow of the air will cause various problems. Firstly, the interior space of the muffle tube is contaminated by dust in the air. The dust comprise SiO.sub.2, Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3 and the like. Among them, Al.sub.2 O.sub.3 will cause devitrification of the preform, and Fe.sub.2 O.sub.3 will cause increase of transmission loss of the optical fiber. Secondly, the inner surface of the carbon muffle tube is oxidized. During oxidation of the sintered body of carbon, it is known that tar or pitch which is used as a binder is firstly oxidized. Therefore, the remaining graphite particles are dropped or splashed and float in the furnace. Since said particles adhere to the surface of the sintered glass preform, the optical fiber fabricated from such the glass preform has many parts with low strength. As a natural consequence, the lifetime of the carbon muffle tube is extremely shortened.
The first measure to prevent such oxidation of the muffle tube is to reduce the temperature to 400.degree. C. or lower at which the carbon is not oxidized during the insertion and removal of a glass preform. However, at such the low temperature, the operation rate of the furnace is greatly decreased. In addition, once the muffle tube is exposed to the air, a considerable amount of oxygen and moisture in the air is adsorbed on the muffle tube since the carbon muffle tube is porous. Then, the oxidation cannot be prevented completely.
As the second measure, a method is described in PCT International Publication No. WO 88/06145. The method comprises once disposing the porous glass preform in a front chamber on the top of the muffle tube and inserting the glass preform into the muffle tube after gas replacement of the atmosphere in the front chamber with an inert gas. The muffle tube equipped with the front chamber is shown in FIG. 4.
The carbon heater 4 and the carbon made muffle tube 3 are provided in the furnace body 5 as shown in FIG. 4. The heating furnace comprises an inlet 6 for introducing a purging nitrogen to the furnace body, an inlet 7 for introducing an atmosphere gas to the muffle tube, supporting rod 2 for the glass preform, the front chamber 11, an exit 14 for exhausting the gas from the front chamber and a partition 16. The glass preform 1 is inserted into the heating furnace.
The insertion of the porous glass preform into the heating furnace shown in FIG. 4 is carried out as follows:
(1) To a rotatable and vertically movable chuck, the porous glass preform 1 is attached through the supporting rod 2. PA1 (2) An upper cover of the front chamber 11 is opened, and the porous preform 1 is lowered into the front chamber 11. PA1 (3) The upper cover is closed, and the interior space of the front chamber is purged with an inert gas (e.g. nitrogen or helium). PA1 (4) The partition 16 which separates the front chamber 11 from the heating atmosphere is opened, and the porous preform 1 is introduced in the heating atmosphere which has been kept at a temperature at which the preform should be thermally treated. PA1 (5) The partition 16 is closed. PA1 (1) The partition 16 is opened. PA1 (2) The preform 1 which has been thermally treated is pulled up from the heating atmosphere to the front chamber 11. In this step, the temperature of the heating atmosphere is not necessarily lowered. PA1 (3) The partition 16 is closed. PA1 (4) The upper cover of the front chamber 11 is opened, and the preform is removed from the chamber 11. PA1 (1) The partition 16 consists of three members so that it takes a considerable time to close or open the partition; PA1 (2) The structure of the partition 16 is rather complicated. In particular, it is difficult to confirm the perfect closing of the split members. Then, there is a great possibility to open the front chamber to the air with an insufficient sealing; and PA1 (3) It takes a long time to replace the interior atmosphere of the front chamber with the inert gas since the volume of the partition portion increases due to the complicated structure of the partition.
The removal of the glass preform from the heating furnace is carried out as follows:
Though the above furnace is superior in preventing oxidation of the muffle tube, an overall length of the heating apparatus is too long and the structure of the partition 16 is complicated. FIG. 5 shows an example of the heating apparatus for thermal treatment of a porous glass preform having an overall length of 800 mm and a seed rod length of 200 mm. In this case, the length from the lower end of the muffle tube to the lower end of the chuck is 6760 mm and the overall length of the apparatus reaches nearly 8000 mm by taking account of spaces for operation and design.
Further, since the partition 16 should be adaptable to both cases where the supporting rod is and is not through the partition, totally three members should be used, two of which are split members each having an partly cut off portion for the supporting rod, and one of which is a member to close an aperture through which the rod is passed.
Then, the procedures for operating the partition 16 is hereinafter described with reference to FIGS. 6 to 8.
FIG. 6 shows a cross-sectional view of the partition in detail when the glass preform 1 is disposed in the front chamber 11, FIG. 7 shows when the glass preform 1 is being inserted into the heating atmosphere of the muffle tube through the partition 16 after the atmosphere in the front chamber is replaced with an inert gas and the partition 16 is opened, and FIG. 8 shows when the glass preform is thermally treated.
To carry out the above three operations, three members, that is, the covering member 72 for covering an aperture and two split members 71 which are operated with two sliding rods 73 should be used.
Then, some problems arise as follows: